During the Sunday-night Democratic debate, Sanders saying recently that the former president's indiscretions were "totally disgraceful and unacceptable." And he was irritated that Mitchell even asked about it.

"That question annoys me," Sanders said.

Sanders complained that the media put too much pressure on him to attack the former secretary of state.

"I can't walk down the street — Secretary Clinton knows this — without being told how much I have to attack Secretary Clinton. Want me to get on the front page of the paper? I make some vicious attack. I have avoided doing that, trying to run an issue-oriented campaign."

"You didn't have to answer it that way, though," Mitchell said, referring to Sanders answer about Bill Clinton's indiscretions.

He added: "His behavior was deplorable. Have I ever said one word about that issue? No I have not. I'm going to debate Secretary Clinton and Governor O'Malley on the issues facing the American people, not Bill Clinton's behavior."